Generally, industrial sites which are essential to manage a flow rate of fluid measure the flow rate within a conduit using a flowmeter. Various types of technologies of measuring an amount of fluid flowing in the conduit have been published. A representative example thereof may include a volumetric flowmeter, an electromagnetic flowmeter, a mass flowmeter, a turbine flowmeter, a differential pressure flowmeter, and the like. Recently, an ultrasonic flowmeter which measures an ultrasonic moving time to obtain a linear average flow velocity and calculate a flow rate has been interested as a technology of supplementing disadvantages of the existing flowmeters. According to a flow measurement principle using the ultrasonic flowmeter, a flow rate may be basically obtained based on an average velocity of fluid and a vertical cross sectional area of the conduit filled with the fluid. This will be additionally described below.
FIG. 1 illustrates a diagram for describing the basic flow measurement principle of the ultrasonic flowmeter, FIG. 2 illustrates a flowing form of fluid within the conduit, and FIG. 3 illustrates the number of sidings corresponding to the number of ultrasonic paths within the conduit and illustrates that respective sidings do not cross each other.
First, as illustrated in FIG. 1, when a conduit having a diameter D is provided with a pair of ultrasonic sensors 1 and 2 by an angle θ, an ultrasonic moving time t between the ultrasonic sensor 1 and the ultrasonic sensor 2 may be defined by the following Mathematical Equation 1 when there is no flow of fluid.
                    t        =                  d          C                                    Mathematical        ⁢                                  ⁢        Equation        ⁢                                  ⁢        1            
In the above Mathematical Equation 1, t represents the ultrasonic moving time, d represents a distance between the ultrasonic sensors 1 and 2, and C represents the ultrasonic moving velocity m/s.
When a fluid flows within the conduit at velocity v, the ultrasonic moving time may be obtained as follows.
First, when an ultrasonic moving direction and the flow direction of fluid are the same, for example, when the ultrasonic wave is transmitted from the ultrasonic sensor 1 and the fluid moves from left to right, the ultrasonic moving time t1 depends on the following Mathematical Equation 2, and
                              t          1                =                  d                      C            +                          v              ⁢                                                          ⁢              cos              ⁢                                                          ⁢              θ                                                          Mathematical        ⁢                                  ⁢        Equation        ⁢                                  ⁢        2            
When the ultrasonic moving direction and the flow direction of fluid are opposite to each other, that is, when the fluid moves from left to right and the ultrasonic wave is transmitted from the ultrasonic sensor 2, the ultrasonic moving time t2 depends on the following Mathematical Equation 3.
                              t          2                =                  d                      C            -                          v              ⁢                                                          ⁢              cos              ⁢                                                          ⁢              θ                                                          Mathematical        ⁢                                  ⁢        Equation        ⁢                                  ⁢        3            
In the above Mathematical Equations 2 and 3, t1 and t2 represent the ultrasonic moving time, d represents a distance between the sensors, C represents the ultrasonic moving velocity m/s, v represents a fluid velocity of the ultrasonic path, and θ represents an installation angle of the ultrasonic sensors 1 and 2.
In the above Mathematical Equations 2 and 3, arranging the fluid velocity v after subtracting 1/t2 from 1/t1, which are reciprocal number of the ultrasonic moving time, becomes the following Mathematical Equation 4.
                    v        =                              d                          2              ⁢                                                          ⁢              cos              ⁢                                                          ⁢              θ                                ⁢                      (                                          1                                  t                  1                                            -                              1                                  t                  2                                                      )                                              Mathematical        ⁢                                  ⁢        Equation        ⁢                                  ⁢        4            
A flow rate Q flowing in the conduit may be basically calculated by a product of the average velocity of fluid by the vertical cross sectional area of the conduit filled with the fluid. In order to convert the fluid velocity v into an average velocity {circumflex over (v)} of fluid flowing in a cross-section of the conduit, the fluid velocity v of the ultrasonic path needs to be divided by a compensating factor. In this case, the flow rate flowing in the conduit may be calculated by the following Mathematical Equation 5.
                    Q        =                              v            k                    ⁢                                    π              ·                              D                2                                      4                                              Mathematical        ⁢                                  ⁢        Equation        ⁢                                  ⁢        5            
The reason of compensating for the fluid velocity v of the ultrasonic path by the compensating factor k in the above Mathematical Equation 5 is that as illustrated in FIG. 1, the flow velocities on the ultrasonic sensors 1 and 2 axes facing each other are the linear average flow velocity but a flow velocity distribution is present in the conduit, and therefore the flow velocity has a difference from the average velocity of fluid flowing in the cross-section of the conduit. Therefore, an error of flow measurement may be reduced only when the linear average fluid velocity v needs to be compensated by the average velocity of fluid flowing in the cross-section of the conduit.
However, in case of using the compensating factor k generally used to compensate for the linear average fluid velocity v, since it is assumed that the measured axis (corresponding to the ultrasonic path) passes through a center of the conduit as illustrated in FIG. 2(a) and the flow velocity distribution has an ideal distribution symmetrical to the measured axis, when the actual flow velocity distribution is biased (asymmetrical) as illustrated in FIGS. 2(b) or (c), the compensation error cannot also but be large. There is a method of increasing the number of axes (referred to as siding) measured to reduce the effect of the compensating error to two or four as illustrated in FIG. 3 to calculate an average fluid velocity. However, since the ultrasonic sensors are arranged so that the respective sidings are arranged in parallel as illustrated in the right of FIG. 3, a general ultrasonic flowmeter illustrated in FIG. 3 may not also measure the fluid velocity between the axes. Therefore, the ultrasonic flowmeter having the arrangement of the ultrasonic sensors as illustrated in FIG. 3 may not also completely compensate for an error due to drift currents. When the overall inside of the conduit is measured by indefinitely increasing the number of sidings, the measurement error may be reduced even though the drift current is present, which is physically impossible. For reference, in FIG. 2, a dashed line within the conduit represents a line connecting the places where the velocities of fluid are the same in the same cross-section.
Meanwhile, one of the error occurring factors in the ultrasonic flowmeter is a vibration of the conduit or a transmitting vibration of the peripheral ultrasonic sensors. That is, even when the vibration generated by peripheral factors affects the ultrasonic sensors 1 and 2 along a wall of the conduit made of metal, the error of the flow measurement occurs, and therefore a method for minimizing an error due to the vibration of the conduit is required.